The first concept of lithium ion battery (LiB) was established in 1962. Soon, rechargeable LiB was proposed by Michael Stanley Whittingham in the 1970s at Exxon, leading to the discovery of a lithium-titanium-disulfide battery. However, the commercialization of the rechargeable LiB using metallic lithium as the anode and air/water sensitive titanium disulfide as the cathode fell apart due to the weak safety of LiM and the high processing cost of titanium disulfide.
Subsequently, reversible intercalation-based graphite and cathodic oxides were developed by Jurgen Otto Besenhard to overcome the aforementioned obstacles and invent practical rechargeable LiB. In 1991, the first commercial LiBs were launched by Sony and Asahi Kasei. This was a revolutionary moment that lead to successful discoveries in portable electronics. Ever since LiB gained wide popularity, the demand for electrical energy storage has soared due to the continuous innovation of portable, everyday electronic devices such as cell phones, music players, speakers, drones, vehicles, and micro/nano sensors. Hence, researchers and scientists are progressively investigating new, advanced energy materials, chemistries and physics for static/non-static energy storage systems to further meet the growing energy demands.
As current commercial LiB technologies have reached a saturation point where only incremental improvements on the electrochemical performance of LiB are reported, new energy materials with different configurations and chemistries are needed to accommodate the ever-increasing energy demand. Thus, rechargeable batteries with LiM anodes and conversion-type cathodes such as lithium sulfur (LiS) and lithium air (LiO) batteries are considered to be the next generation of LiB due to their high energy densities and practicalities.
The sulfur and oxygen cathodes have theoretical specific energy densities of ˜2,600 Wh/kg and ˜11,400 Wh/kg, respectively. These values are almost 10 times higher than that of current LiB (˜360 Wh/kg for C/Co2O4). Furthermore, LiM provides an exceptionally high theoretical specific capacity of ˜3,860 mAh/g with very low negative redox potential (−3.04V vs S.H.E.) and a density of 0.59 g/cm3. Moreover, the intercalation-type graphite anode exhibits a lower theoretical specific capacity (<372 mAh/g) with a higher negative redox potential and density. This means that the gravimetric energy density of the conventional LiB could increase dramatically by replacing the graphite anode with metallic lithium. These desirable properties of the LiM anode and conversion-type cathodes could provide a promising pathway to overcoming the high energy demand challenges in the future if LiS and LiO batteries can be commercialized soon.
Despite their assuring electrochemical characteristics, several prominent challenges are need to be resolved in order for LiM batteries to be fully commercialized. The central issue is the reversibility of the deposition/dissolution of the Li ions. The high reactivity and nonuniform deposition of Li cause various interrelated problems such as thermal runaway, decomposition of electrolytes, and loss of Li. The uneven deposition of Li ions onto the anode during the charging process leads to the formation of metallic protrusions that grow and eventually short-circuit the cell by piercing the separator. This is a serious safety concern because such a short circuit can produce large amounts of heat and sparks that may ignite the electrolyte that is comprised of flammable organic liquids. Other challenges associated with LiM battery are coulombic instabilities and side reactions that result in the battery's low capacity and short cycle life. Such instabilities arise from the continuous reaction between the LiM and the electrolyte. In each successive charge and discharge cycle, the SEI breaks down and a new SEI is formed by the reaction of the electrolyte with the exposed Li. This unwanted process leads to the continuous degradation of the electrolyte and the formation of insulating species inside the battery. These fragmented troubles hinder the commercialization of the LiM batteries especially due to safety concerns. Hence, it is essential to first create a stable solid electrolyte interface (SEI) and/or mechanical protection layers on the active lithium surface to provide uniform anchoring points that will enable the stable deposition and dissolution of the Li ion. In this scenario, formation of the ramified lithium dendrites could be effectively suppressed.
First, Cui and co-workers at Stanford University proposed isolating LiM from the electrolyte by placing an interconnected hollow carbon sphere film (thickness ˜200-300 nm) in-between so as to provide an electrochemically and mechanically stable artificial SEI layer called “Hard-Film” that could block the lithium dendrites. Furthermore, Archer and co-workers at Cornell University demonstrated that LiF coated Li can retard Li dendrite growth or become dendrite-free Li by forming a stable SEI layer. Other effective chemical additives and soft SEI films have been proposed to postpone or suppress Li dendrites; however, simplified procedures of film fabrication that are also cost effective need to be developed in order to improve the practicality and safe utilization of LiM as an anode.
LiM anode protection becomes even more challenging when conversion-type cathodes are involved. Archer and Nazar demonstrated the use of highly reversible LiS batteries by employing hollow carbon nanospheres and highly ordered nanostructured carbon (i.e. CMK series) as an effective sulfur host in 2011 and 2009. Since then, interest in the LiM anode and sulfur cathode have resurged. For LiS batteries, the redox reaction between lithium and sulfur (16Li+S8↔8Li2S) occurs spontaneously and is fully reversible. Moreover, sulfur is low-priced and earth-abundant, which provides an additional incentive for the development of LiS electrochemical storage technology. However, the development of a practical LiS battery has been postponed by fundamental problems associated with multiple transport and thermodynamics. First, sulfur suffers from poor electrical conductivity (5×10−30 S cm−1 at R.T.) and produces a discharge product (Li2S) that has an insulating characteristic. Second, the volume of sulfur increases by ˜80% when it is fully lithiated. Furthermore, the redox reaction product, Li2S, is always accompanied by the formation of various dissolvable intermediate species called lithium polysulfides (LiPS Li2Sn, 2<n<8), which create challenges in active material loss and reutilization. LiPS are highly soluble in organic electrolytes and therefore cause the loss of active materials in the cathode. Once present in the electrolyte, the LiPS species can diffuse through the separator and reach the Li anode, establishing an internal shuttling pathway between the Li anode and sulfur cathode. This phenomenon is known as LiPS shuttling. During shuttling, the dissolved LiPS, especially high-order LiPS, gets reduced at the surface of the Li anode and passivates the anode surface. As a result, rapid capacity fading, poor cycling lifetime, low coulombic efficiency, and chemical shorting are observed in the LiS batteries. To resolve the issue of the anode engaging in a side reaction with LiPS, the LiNO3 additive is found to be very effective in protecting the LiM as it provides a protective passivation layer on the surface. However, the electrolyte additives in the LiS battery do not provide solutions for lithium dendrites. Therefore, it is urgently required that the electrochemically stable LiM anode with conversion-type (especially sulfur) as well as intercalation-type cathodes are desired to fabricate high energy density secondary batteries.
In light of these grave challenges, this invention intends to present a facile way of preparing ultra-thin artificial SEI layer films at the atmospheric condition via LBS method. Graphene is considered to be the most promising artificial SEI layer as graphene allows for the rapid diffusion of Li ions. This indicates that graphene is capable of transferring Li ions in 3 dimensional spaces and providing effective electrolyte/anode separation to prevent problematic side reactions and Li dendrites. Also, the high modulus of graphene (0.5 TPa) could block the lithium dendritic proliferation. There are several processing techniques to prepare graphene films such as dip-coating, spray coating, spin coating, inkjet printing, doctor blade, electrodeposition, vacuum filtration, drop casting, interfacial deposition, conventional Langmuir-Blodgett, and LBL assembly. Despite these various techniques for the graphene processing, there is no fast, facile, simple, and cost-effective method that yields high quality ultra-thin graphene films with precise thickness control and large-scale adoptability. Additionally, nano-ceramic materials are known as suitable protection materials for Li dendrites and separators as they have high modulus and thermal conductivity. However, the high interfacial impedance of the ceramic coatings on the anode side impedes the high power density of LiM batteries. To overcome this serious issue, a simply synthesized lithium terminated sulfonated ceramic is used as the anode protection layer. With this functionalization, a lower impedance and more stable lithium deposition/dissolution are achieved. Consequently, the functionalized ceramic is paired up with the graphene film to create multifunctional artificial SEI films. These films are then transferred to lithium to fabricate a stable and safe metallic Li anode for LiM batteries.